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cos^4 x - sin^4 x = 1 - 2sin^2 x

Prove

3 answers

Work with only one side to get it to match the other. It's critical you alter only ONE side.

What progress have you made, and where are you having problems?
I am completely stumped. I've never worked with ^4 before and I don't know how to factor it out.
Break cos^4(x) into cos^2(x)*cos^2(x).
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